FR2498372A1 - DIRECT HEATING CATHODE, METHOD FOR MANUFACTURING SAME, AND ELECTRONIC TUBE INCLUDING SUCH A CATHODE - Google Patents

Abstract

THE OBJECT OF THE INVENTION A CATHODE WITH THERMO-ELECTRONIC EMISSION WITH DIRECT HEATING. IT INCLUDES A PYROLYTIC GRAPHITE SUPPORT 1 AND AN EMISSIVE COATING 2 CONSISTS OF A MIXTURE OF TUNGSTENE AND LANTHAN OXIDE, THE TUNGSTENE BEING TRANSFORMED IN ITS SURFACE PART 3 INTO TUNGSTEN HEMICARBIDE. THE INVENTION APPLIES IN PARTICULAR TO HIGH FREQUENCY TUBES OF THE TRIODE, TETRODE OR PENTODE TYPE.

Description

DIRECT HEATING CATHODE, METHOD FOR MANUFACTURING SAME,

  AND ELECTRONIC TUBE INCORPORATING SUCH A CATHODE.

  The subject of the present invention is a cathode for a high frequency electron tube, and more particularly a cathode with thermoelectronic emission with direct heating. She also has

  object is an electron tube comprising such a cathode.

  In the high frequency electronic tubes of the triode, tetrode or pentode type, which comprise a cathode, an anode and one, two or three grids, it is advantageous to produce the grids made of pyrolytic graphite, a material known for its mechanical qualities.

and thermal.

  However, in these same tubes the cathodes are generally

  made of tungsten or thorium tungsten wire for

  reasons of thermo-electronic emissivity, the temperature of

  tion is then between 1900 and 2000 @ K. There then arise, in operation, mechanical problems due to the difference in thermal behavior of the materials, problems imperfectly solved by expensive mechanical assemblies. It has been proposed to avoid thermomechanical problems inside the tube while ensuring a good thermo-electronic emissivity by introducing direct heating from a pyrolytic graphite support and depositing on the surface of the graphite a material emitting at a lower temperature than tungsten or thoriated tungsten such as lanthanum hexaboride LaB6 for example. Such a

  This structure makes it possible to obtain electronic

  between 1400 and 15000C. However, a disadvantage of emissive materials such as Lanthanum hexaboride is their high chemical activity with respect to hot graphite, which can lead to the destruction of the cathode. It is therefore necessary to introduce an intermediate layer between the graphite and the hexaboride of

  Lanthanum forming diffusion barrier between these two materials.

  The present invention relates to a heating cathode

  direct working at the same temperature as the cathode in hexa-

  lanthanum boride, but not requiring an intermediate layer

  between the graphite and the emissive layer.

  With respect to a cathode commonly used in the prior art, the main advantages involved are:

  - a lower operating temperature.

- better mechanical strength.

  With respect to a lanthanum hexaboride cathode, the advantage

  is the removal of the intermediate layer.

  The cathode according to the invention comprises a pyrolytic graphite support and an emissive coating composed of a mixture of

  tungsten and rare earth oxide (eg lanthanum oxide)

ple).

  The emitting layer can be carburized on the surface to improve

improve the program.

  Other objects, features and results of the invention

  - will appear from the following description appended by the single figure

  which represents a sectional view of an embodiment of the

cathode according to the invention.

  This comprises a support 1 of pyrolytic graphite having a thickness of about 200p, on which is deposited, by plasma or sputtering, or by any other means known to those skilled in the art, a homogeneous layer ( 2) a mixture of tungsten and lanthanum oxide the latter being in proportions of between 0.5% and 10%, the thickness of the layer (2) can be understood

between 50 and 100p.

  The tungsten of the emitting layer can be converted in its surface portion (3), to a thickness of 10 to 20 P, into tungsten halide W2C. This conversion is carried out in a conventional manner by heating the cathode in hydrocarbon vapors at a temperature of about 1800 ° C. In another variant, the tungsten carbide can be coded with tungsten and lanthanum oxide, proportion ranging from 10% to 50% of carbide, from 0.5 to 10% of lanthanum oxide,

  the balance being made by tungsten. This variant makes it possible

  undermine the carburization process of tungsten.

Claims (7)

  1. cathode with direct heating, characterized in that it comprises a support (1) cut out pyrolytic graphite heated by Joule effect, and an emissive coating consisting of a layer (2)   composed of a mixture of tungsten and rare earth oxide.   2. Cathode according to claim 1, characterized in that the rare earth oxide constituting the layer (2) is lanthanum oxide. 3. Cathode according to claim 1, characterized in that the upper part (3) of the layer (2) consists of hemicarbide of tungsten.   4. Cathode according to claim 1, characterized in that the proportions of rare earth oxide constituting the layer (2) are   between 0.5% and 10% of the weight of the mixture.   5. Cathode according to claim 1, characterized in that   the thickness of the layer (2) is between 50, u and 100p.   6. A method of manufacturing a cathode according to the claims   1 to 3, comprising the following successive steps: a) Mixture of tungsten powders and rare earth oxides,   b) Pressing the mixture under a pressure of about 3 tons   nes / cm2, c) Sintering at a temperature of 20001C approximately, d) depositing said mixture (2) by sputtering on a pyrolytic graphite support (1),   e) heating at a temperature of about 18000 C under pres- reduced hydrocarbon fraction.   7. Electronic tube for microwave, comprising a cathode, anode and at least one gate, characterized in that   that said cathode is a cathode according to one of claims 1 to 6.